Controlled expansion alloys are useful in many applications, most of which, to date have not represented major markets for metal. For example, the Eiselstein and Bell, U.S. Pat. No. 3,157,495 is directed to a nickel-coablt-iron alloy having controlled thermo-elastic properties up to elevated temperatures. The alloys provided in accordance with this patent are age-hardenable and develop excellent strength and ductility values at ordinary temperatures. In addition, the alloys were found to have highly useful strength properties at elevated temperatures and had long rupture lives at temperatures up to 1000.degree. F. although quite low ductility in properties were then observed.
U.S. Pat. No. 3,705,827 reports on a heat treatment procedure for heat treating age-hardenable chromium-free and chromium-containing nickel-iron alloys. Development of high strength in the age-hardenable alloys together with useful rupture life at temperatures on the order of 1150.degree. F. are reported in this patent.
U.S. Pat. No. 4,006,011 is directed to an essentially chromium-free, age hardenable, nickel-cobalt-iron alloy capable of providing high strength at ordinary temperatures and having useful stress rupture properties at certain elevated temperatures, such as 1150.degree. F.
Recently, an interest has been expressed in alloys having controlled expansion characteristics up to temperatures in the order of 1000.degree. F. or 1100.degree. F. Thus, it has been considered that various parts used in aircraft gas turbine engines, such as rings, seals, casings, nozzle supports, etc. could usefully be produced of nickel-iron or nickel-cobalt-iron alloys having controlled expansion characteristics even though the alloys are ordinarily regarded as being deficient in oxidation resistance in oxidizing atmospheres at temperatures encountered in the hot zones of aircraft gas engines. Further pursuit of the requirements properly to be imposed upon such alloys in aircraft gas engine applications has developed the fact that the alloys and the heat treatments therefore which have been provided to date are still subject to deficiencies, namely inadequate notch strength at temperatures on the order of 1000.degree. F. Thus, even the alloys provided in accordance with the teachings of U.S. Pat. No. 4,200,459 which are nickel-iron-cobalt alloys having controlled low aluminum contents were still deficient in notch strength at temperatures of 1000.degree. F. or thereabouts when subjected to the age-hardening treatment schedules disclosed.
Progress in the development of the alloys have now lead to heat treatments applicable to alloys akin to those disclosed in U.S. Pat. No. 4,200,459 which are capable of rendering the alloys in a condition wherein they have adequately high tensile strength and ductility together with adequately high notch strength at the temperatures of interest to aircraft engine designers, e.g. 1000.degree. F. Along with the developing need for high notch strength at elevated temperatures, purchasers of controlled expansion alloys have become increasingly concerned about the availability and cost of the cobalt used in these alloys. A demand has accordingly been created not only for alloy articles and parts having mechanical properties newly recognized as being necessary but also for alloys which would be essentially cobalt-free while still retaining useful expansion characteristics. It is the purpose of the present invention to supply the foregoing complex of requirements in a practical way.